The invention relates to improvements in knee joint prostheses. More particularly, the invention relates to improvements in devices which are used in such prostheses to establish an articulate connection between the femoral and tibial components. Still more particularly, the invention relates to improvements in knee joint endoprostheses of the type described, claimed and shown in commonly owned copending patent application Ser. No. 07/800,327 filed Nov. 29, 1991 by Eckart Engelbrecht and Elmar Nieder for "Knee joint prosthesis".
Knee joint prostheses can be divided into two main categories, namely partial and full prostheses. Each of these prostheses is intended to enable a patient to walk again or to enhance the ability of a patient to walk, and each of these prostheses can be utilized to at least partially restore the integrity of a knee joint which is damaged as a consequence of a disease and/or as a result of trauma.
A partial prosthesis restores the surfaces of femoral and/or tibial condyles and cooperates with the ligaments (including the medial, lateral and crucial ligaments). A properly implanted partial prosthesis cooperates with the ligaments in the transmission of loads (mechanical functions) as well as in regulation of various movements. A prerequisite for proper functioning of a partial prosthesis is that the ligaments be at least substantially intact, i.e,, such prostheses should not be called upon to compensate for certain diseases or defects of the knee joint which involve damage to the ligaments. The four most common types of such diseases or defects are genu varum, genu valgum, genu flexum and genu laxum. Decompensation would entail shrinkage or overextension of passive and active stabilizers in a knee joint or a combination of both. Ligaments constitute the passive stabilizers of a knee joint, and the active stabilizers include the tendons of muscles which extend across the knee joint. Stabilizers can be classified as medial, lateral or dorsal stabilizers (provided that the stretching system is disregarded). The functioning of a partial prosthesis is unsatisfactory, and its useful life is too short, if the active and/or passive stabilizers are damaged in a manner as outlined above, i.e., if such stabilizers have undergone shrinkage and/or overextension. The reason is that a partial prosthesis constitutes a totally uncoupled system. A partial prosthesis which employs a plane plateau (also called sled type prosthesis) can be implanted only when the stabilizers are entirely intact. A partial prosthesis which utilizes a slightly grooved or otherwise uneven plateau can compensate, to a certain extent, for minor weakening of active and/or passive stabilizers.
A total prosthesis must be implanted in the event of destabilization of passive and/or active stabilizers. Such prostheses take over not only the mechanical functions (i.e., transmission of loads) but also the regulation of movements of the femur and tibia relative to each other. A total prosthesis does not afford to the knee a freedom of movement matching that which is afforded by a partial prosthesis. On the other hand, a properly implanted total prosthesis imparts to the knee joint a higher degree of stability than a partial prosthesis even though the passive and/or active stabilizers of the repaired knee joint are defective.
Total prostheses include so-called hinge- or flexion-type prostheses wherein the femoral and tibial components can perform a single movement relative to each other, namely pivoting about a predetermined axis which is defined by the shaft or pintle of the hinge serving to articulately connect the two components. The single form of movement which is permitted by such types of prostheses is known as monocentric flexing. Another class of total prostheses embraces the so-called rotational prostheses which enable the tibial and femoral components to perform any movements afforded by a flexion-type prosthesis plus rotation of the tibia relative to the femur. Tibial rotation serves to protect the connections between the shanks of the two components and the bones into which the shanks are implanted as well as the bones themselves because the moments which develop as a result of rotation about the longitudinal axis of the respective leg are transmitted to the remaining intact soft parts in the region of the knee joint. For the above reasons, the useful life of connections between the implanted shanks of femoral and tibial components of a rotational prosthesis and the respective bones is longer than that of connections in a flexion-type total prosthesis. Moreover, the likelihood of fracturing the tibia and/or the femur in the event of trauma is less pronounced than if the implanted prosthesis is a flexion-type prosthesis. It is a well known fact that the useful life of connections between the shanks and the bones into which the shanks are implanted is longer if the total prosthesis enables the femur and the tibia to perform flexural as well as rotational movements relative to each other. Therefore, a rotational prosthesis is normally preferred over a purely flexural prosthesis. On the other hand, limitations upon the usability of a rotational prosthesis are imposed by the nature of deformity of the affected knee joint.
Genu flexum (which could be termed an overly bent knee) is a deformity which prevents the afflicted knee joint from permitting the tibia to move to a fully extended position of substantial alignment with the femur. The reason for the development of such deformity is stretching of the rear (dorsal) capsula and/or shrinkage of tendons which are active during flexing of the tibia. In order to remedy the situation, a surgeon will detach the rear capsula at the point of connection to the dorsal side of the thigh. In most cases of a contraction of a tendon, such operative treatment suffices to ensure that, once the rotational prosthesis is implanted, the knee joint regains its ability to permit full extension or stretching of the shin relative to the thigh. If the genu flexure is very pronounced, i.e., if mere detachment of the rear capsula does not suffice to remedy the situation, this indicates a pronounced shrinkage of the tendons which are necessary to flex the shin relative to the thigh. The possibilities to remedy such defects are twofold, namely: One can resort to so-called lengthening (Z-plastic) of affected tendons or to stepwise resection of the tibia. The lengthening of tendons is a time-consuming operation which necessitates additional surgery and lengthy postoperative immobilization of the knee which is contrary to the presently preferred procedure of mobilizing a patient as soon as possible following implantation of a knee prosthesis. Therefore, specialists in the field of implantation of knee joint endoprostheses prefer to resort to stepwise recession until the thus corrected knee joint permits full stretchability of the shin. If such full stretchability is achieved only upon extensive tibial resection, this results in mere pseudostability in extended position of the shin, and such pseudostability is attributable to stretching of the remaining shortened dorsal stretching tendons. If a patient who has undergone such treatment decides to bend her or his shin, all of the structures which extend across the knee, the stretching system and the remaining capsular band sleeve or its scarred regenerate are too long. This brings abut the danger of subluxation or total dislocation in the joint between the shaft and the sleeve of the rotational prosthesis, both likely to affect the functioning and/or the tissue in the region of the knee. Therefore, when the required treatment involves stepwise resection of the tibia, the surgeon in charge will normally decide to implant a flexion-type prosthesis which merely permits flexing of the shin and thigh relative to each other about a single axis.
Genu varum (known as bowleg) is attributable primarily or exclusively to loss of the level of medial femoral and tibial condyles. Decompensation of passive and active medial and lateral stabilizers is infrequent and develops at a more advanced age, i.e., shrinkage of medial structures and overextension of lateral structures are infrequent and develop only when the bowleg becomes very pronounced. Therefore, the possibilities of implanting a rotational prosthesis in the knee of a bowlegged patient are practically unlimited because the ligaments which have undergone contraction or excessive stretching are detached in the course of the normal operative procedure and the scarred regenerate of lateral ligaments and capsula normally suffices to establish and guarantee intimate contact between the femoral and tibial components. The need for implantation of a flexion-type knee joint endoprosthesis in the case of a bowleg arises only when the genu varum develops simultaneously with highly pronounced genu flexum, namely when the aforedescribed extensive resection of the tibia is necessary to correct the flexural component.
Genu valgum (known as knock-knee) is attributable to premature decompensation of stabilizers. Such defects can be remedied, practically at any stage, by implantation of a rotational prosthesis. Decompensation of stabilizers in a genu valgum does not, by itself, limit the usefulness of a rotational prosthesis because the lateral ligaments at the femoral condylum must be detached anyway and the scarry regenerated lateral ligament sleeve guarantees the establishment of an intimate contact between the components. However, genu valgum can involve complicating additional problems. The active stabilizers (musculus bicepts and its tendon, tractus iliotibialis, musculus peblitius and its tendon and the lateral gastocnemius head) predominate at the exterior of the knee joint, and such active stabilizers turn outwardly in inclined position of the tibia. This is their normal additional function. However, if such active stabilizers and/or their tendons happen to shrink as a result of premature decompensation in the case of genu valgum, this is further complicated by improper positioning of the shin upon completion of turning in an outward direction. This can be remedied by resorting to the aforediscussed (Z-plastic) treatment (lengthening)of the active stabilizers and their tendons. Such treatment is expensive, as in the case of flexural tendons, and necessitates postoperative immobilization of the knee. Therefore, specialists in the relevant field of surgery prefer to remedy a genu valgum with the aforedescribed side effect (improper positioning of the shin upon turning in outward direction) by notching or even removing the active outwardly turning shrunk stabilizers and by implanting a flexion type prosthesis in order to thus compensate (a) for weakening of the knee due to removal of stabilizers and (b) for the eventually remaining tendency of the shin to assume an improper position upon turning in an outward direction.
Genu laxum is a deficiency which is attributable to extreme overextension of all stabilizers. Therefore, such problem cannot be overcome by implanting a total prosthesis of the type known as rotational prosthesis, i.e., the only remedy at the disposal of a surgeon is to implant a hinge- or flexion-type prosthesis in order to keep the tibial and femoral components together.
The aforediscussed limitations regarding the utilization of various types of knee prostheses apply essentially for the most frequently occurring problems, namely the idiopathic (innate) arthrosis. In the case of rheumatic arthritis, which is the next most frequently occurring class of illnesses, and which can also necessitate the implantation of a knee joint prosthesis, the aforediscussed considerations regarding the utility or desirability of various types of knee joint prostheses are even more pronounced because the muscles of a patient who is suffering from rheumatism are weakened anyway. In many instances, rheumatic arthritis entails the development of genu valgum with the aforediscussed additional complication (improper positioning of the shin upon turning in an outward direction). Therefore, the implantation of a simple flexion-type knee joint prosthesis is preferred under such circumstances or most by many surgeons.
Knowledge of various afflictions of knee joints has been greatly expanded during the last years. Furthermore, specialists in this area have determined, even more reliably, that extensive examinations with assistance from X-ray equipment and/or other recently developed sophisticated apparatus still cannot guarantee optimal advance selection of the most satisfactory prostheses to be implanted into the knees of patients suffering from the aforediscussed afflictions. In other words, it is not possible to predictably plan an operation which involves the implantation of a partial or total knee Joint prosthesis prior to actual start of surgery. Thus, the surgeon must inspect an afflicted knee in the course of an operation in order to select a proper prosthesis which is best suited for implantation into the body of a patient on the operating table. For example, if the illness to be remedied is genu flexum, the surgeon must first detach the rear capsula prior to reaching a decision regarding the amount of bone tissue to be removed from the tibia. This, in turn, will enable the surgeon to decide which of the total prostheses is called for under the existing circumstances. In other words, a hospital or a clinic specializing in operations of knee joints must maintain a variety of prostheses in order to make sure that a proper prosthesis will be available if the ultimate decision regarding the type of prosthesis to be implanted departs from the tentative decisions which were reached upon completion of preliminary examination of the afflicted knee joint.